Heretofore, there is an IEEE (Institute of Electrical Electronics Engineers) 1394 standard as bus standards for transferring multimedia data with high speed in real-time, and various kinds of AV devices which can mutually transmit data through a network using digital serial buses under the IEEE1394 standard have been developed.
In such network system, provided are an isochronous transfer mode for transmitting real-time data and an asynchronous transfer mode for transmitting control commands without fail.
In this network system, therefore, a logical connection (hereinafter, referred to as logical connection) should be established as a signal transfer path between a source and a sink to transmit real-time data, the source generating and outputting the real-time data, the sink receiving the real-time data outputted from the source.
On the other hand, in this network system, the AV devices connected to the network system can be controlled by transmitting AV/C (Audio/video control) command•transaction•set (A/V Command Transaction Set) (hereinafter, referred to as AV/C command) as a control command.
By the way, there exist various plugs for both ends of logical connections, and the plugs include a subunit•plug (Subunit Plug) for transmitting signals inside a device, a plug•control•register (Plug Control Register) (hereinafter, referred to as PCR) for inputting/outputting digital signals between devices, and an external•plug (External Plug) for inputting/outputting analog signals between devices.
As the subunit•plug, there exist a subunit•source•plug (Subunit Source Plug) for output and a subunit•destination•plug (Subunit Destination Plug) for input. As the PCR, there exist an oPCR (Output Plug Control Register) for output and an iPCR (Input Plug Control Register) for input. As the external•plug, there exist an external•output•plug (External Output Plug) for output and an external•input•plug (External Input Plug) for input. In this connection, plugs for output are referred to as output plugs and plugs for input are referred to as input plugs in the following description.
In addition, as a logical connection, there exist an internal device connection and an interdevice connection, and the internal device connection indicates a signal path existing inside a device and the interdevice connection indicates a signal path between a source and a sink.
Further, the interdevice connections have various forms. That is, the interdevice connections include a point•to•point (Point-to-Point) connection (hereinafter, referred to as PtoP connection) in which one oPCR and one iPCR are connected on one isochronous channel (hereinafter, referred to as channel).
In addition, the interdevice connections include a broadcast•out (Broadcast out) connection (hereinafter, referred to Bout connection) in which one oPCR is connected to one channel, and a broadcast•in (Broadcast in) connection (hereinafter, referred to as Bin connection) in which one iPCR is connected to one channel, and these Bout connection and Bin connection are referred to as broadcast (Broadcast) connection.
In addition, the interdevice connections include an external•connection in which an external•output•plug and an external•input•plug are connected.
A procedure to establish and cancel such logical connection is standardized by IEC (International Electrotechnical Commission) 61883-1.
It should be noted that the establishment of one interdevice connection in a network system requires a channel and an isochronous band (hereinafter, referred to band), and the channel and band are referred to as isochronous resource (hereinafter, referred to as resource).
By the way, in such network system, a command set called AV/C connection•and•compatibility•management (AV/C Connection and Compatibility Management) (hereinafter, referred to as CCM) is provided to mutually communicate information for establishing a proper logical connection using a control command which is communicated between a source and a sink at the time of establishing a logical connection.
Now, explanation will be made on how to establish a signal path, which is a logical connection, using the CCM. The internal device connection can be established using a signal•source (SIGNAL SOURCE) command provided under the CCM, and the interdevice connection can be established using an input•select (INPUT SELECT) command provided under the CCM.
The input•select•command defines four sub•functions (sub functions): connect (CONNECT); path•change (PATH CHANGE); select (SELECT); and disconnect (DISCONNECT), and one of them should be specified when an input•select•command is sent. Now, a concrete explanation will be made on a case in which a sink accepts an input•select•command, that is, returns an accepted (ACCEPTED) response.
The sink establishes an interdevice connection with a designated source in the case of receiving CONNECT, and in the case of receiving PATH•CHANGE, it basically establishes an interdevice connection with a designated source, but it is able to reject the establishment of the interdevice connection, that is, to return a rejected (REJECTED) response if it does not select the designated source.
Further, when the sink receives SELECT, it selects a designated source and in this case, the sink can decide by itself whether to establish an interdevice connection. When the sink receives DISCONNECT, it disconnects a designated interdevice connection.
Under the CCM, a source communicates information on the source to a sink by transmitting an input•select•command, so that the sink establishes an interdevice connection with the source based on the information. In this case, in the network system, the sink which is a transmission target of the input•select•command should be recorded in the source in advance. In the CCM, an output•preset (OUTPUT PRESET) command is prepared for realizing such function. In addition, in the CCM, an input•select•command is used to record a source in a sink.
In such network system, based on the CCM, an internal device connection is established by a device itself and an interdevice connection is established by a sink. In this connection, the internal device connection can be controlled from the outside with a signal•source•command.
By the way, in the network system using the CCM, a source sends an input•select•command to a sink with information on an output plug of the source (hereinafter, referred to as output plug information) attached to the input•select•command. The sink establishes an interdevice connection based on the output plug information given in the received input•select•command.
In addition, in the network system, when the sink establishes an interdevice connection, the sink directly establishes the interdevice connection based on output plug information of default.
Furthermore, in the network system, when a controller which is an AV device other than sources and sinks establishes an interdevice connection, the controller sends an input•select•command to a sink with output plug information of default attached to the input•select•command. The sink establishes an interdevice connection based on the output plug information given in the received input•select•command.
By the way, in the network system adopting the CCM, an auto-play processing has been developed to establish an interdevice connection between a source and a sink depending on the change of the state of the source.
Specifically, the controller makes the source register the sink by transmitting an output•preset•command to the source. If a trigger, such as the change to a replay mode, occurs in the source thereafter, the source transmits an input•select•command to the sink. The sink receives the input•select•command, and establishes an interdevice connection based on the information stored in the input•select•command.
In addition, in the network system adopting the CCM, path information notification processing by a source has been developed as a technique to establish an interdevice connection without a controller.
Specifically, when an appropriate output plug of the source is changed, the source notifies each of all registered sinks of new output plug information by transmitting an input•select•command in which the subfunction•field indicates PATH•CHANGE, to all the sinks. Each sink receives the input•select•command and establishes an interdevice connection with an appropriate input plug based on the output plug information stored in the input•select•command.
By the way, in a conventional network adopting the CCM, the selection condition of AV devices may be changed automatically without user operation, which is unnatural and unpreferable operation for a user.
A concrete explanation will be made on such operation hereafter. It is assumed that a network system is constructed by connecting an STB (Set Top Box), VCR (Video Casette Recorder), and TV (Television) to an IEEE1394 serial bus, the TV operates as a controller and a sink, and the STB and VCR operate as sources.
The TV firstly operates to set the VCR for auto-play processing. That is, the TV, which operates as the controller, transmits an output•preset•command to the VCR serving as the source to register the TV as a sink in the VCR serving as the source.
When the VCR serving as the source starts playback operation in response to user operation thereafter, it transmits an input•select•command to the TV serving as the sink. The TV serving as the sink receives this input•select•command, and then establishes an interdevice connection with the VCR.
Sequentially, the TV operates as the controller to register the STB as a source to change images to be displayed on a display from images being outputted from the VCR to images to be outputted from the STB. Specifically, the TV which operates as the controller registers the STB as a source in own functional blocks operating as the sink. In this case, the interdevice connection between the VCR and TV is cancelled.
Next, the TV operating as the controller transmits an output•preset•command to the STB to register the TV as a sink in the STB serving as the source. If the STB serving as the source is now outputting AV data, it transmits an input•select•command to the TV serving as the sink. When the TV serving as the sink receives this input•select•command, it establishes an interdevice connection with the STB.
Now, it is assumed that such change of condition that an appropriate output plug is changed occurs in the VCR, like a case where AV data to be outputted is changed on a tape from digital signals to analog signals, for example. In this case, since the VCR serving as the source has registered the TV as a sink, it notifies the TV of the new output plug information by transmitting an input•select•command in which the subfunction•field is PATH•CHANGE.
When the TV serving as the sink receives this input•select•command, it is designed to establish an interdevice connection with the VCR based on the output plug information stored in the input•select•command. In this case, however, the TV changes images being displayed on the display from output images of the STB to output images of the VCR without user operation, which is unnatural operation unintended by a user.